Actually TeamSpeak works very well.
In the example I have pictured you can see how it is divided into Headquarters, Companies, Platoons and vehicles. Each platoon level and higher has its own frequency
and whispers can be set to the frequency rather than to a specific person. You just need to assign push-to-talk buttons for each frequency. Players can turn on, or off, any of the frequencies that they to listen to, or ignore.
At the platoon level every one is usually on the platoon frequency but individual vehicles are provided for multi crewing situations.
All of this needs to be set up beforehand by the person responsible but from what I can infer it is not difficult to do.